The present invention is directed, in general, to process control systems and, more specifically, to a process control system containing Proportional, Integral, Derivative (xe2x80x9cPIDxe2x80x9d) controllers.
Many process facilities (e.g., a manufacturing plant, a mineral or crude oil refinery, etc.) are managed using distributed control systems. Typical contemporary control systems include numerous modules tailored to monitor and/or control various processes of the facility. Conventional means link these modules together to produce the distributed nature of the control system. This affords increased performance and a capability to expand or reduce the control system to satisfy changing facility needs.
Industrial control systems often employ feedback controllers for controlling the operation of one or more operating units of the system such as a heater, a pump, a motor, a valve, or a similar item of equipment. In a feedback controller a command is sent to the feedback controller that represents a desired value or setpoint (xe2x80x9cSPxe2x80x9d) for a process variable (e.g., a desired pressure, a desired temperature, a desired flow rate). A feedback signal is also sent to the feedback controller that indicates the actual value of the process variable (xe2x80x9cPVxe2x80x9d) (e.g., the actual pressure, the actual temperature, the actual rate of flow). An error signal is calculated utilizing the difference between the setpoint (xe2x80x9cSPxe2x80x9d) command and the feedback signal that indicates the actual value of the process variable.
From the error signal, the feedback controller calculates a change command to change the current setting of the operational unit. For example, if the operational unit is a motor, the change command would cause the speed of the motor to change (either increase or decrease) in order to cause the actual value of the process variable to more closely approach the desired setpoint value for the process variable.
In a simple feedback controller, the change command is proportional to the error signal. In more complex feedback controllers, the change command may be a more complex function of the error signal. The relationship between the error signal and the change command greatly affects the characteristics of the control system. These characteristics include (a) the xe2x80x9cresponse timexe2x80x9d of the system (i.e., how fast the operational unit responds to the new change command); (b) the xe2x80x9covershootxe2x80x9d of the system (i.e., how much the operational unit initially exceeds its new setting); and (c) the xe2x80x9cdamping ratioxe2x80x9d of the system (i.e., how long the output values of the operational unit oscillate before eventually stabilizing at the new setting).
Industrial control systems often employ a type of feedback controller known as a Proportional, Integral, Derivative (xe2x80x9cPIDxe2x80x9d) controller. PID controllers are capable of calculating a variety of functional relationships between an error signal and a change command signal in a feedback control system.
A PID controller may be used to calculate a functional relationship between an error signal and a change command signal that minimizes the time that the control system takes to reach a stable state following a change command signal. PID controllers are capable of operating in three modes. The modes are the Proportional mode, the Integral mode, and the Differential mode. PID controllers generate a proportional-integral-differential function that is the sum of (a) the error signal times a proportional gain factor (xe2x80x9cP gainxe2x80x9d), and (b) the integral of the error signal times an integral gain factor (xe2x80x9cI gainxe2x80x9d), and (c) the derivative of the error signal times the derivative gain factor (xe2x80x9cD gainxe2x80x9d). An appropriate selection of the three gain factors (xe2x80x9cPxe2x80x9d, xe2x80x9cIxe2x80x9d and xe2x80x9cDxe2x80x9d) must be made to calculate a transfer function that will result in a desirable system response. Selecting the three gain factors is sometimes referred to as xe2x80x9ctuningxe2x80x9d the PID controller.
In a PID controller the integral mode will continue to integrate the error as long as the error is not zero. This can cause the output of the PID controller to increase well beyond the acceptable output limits of the PID controller. When this occurs, the PID controller is said to be xe2x80x9cwound upxe2x80x9d or is said to be in a xe2x80x9cwind upxe2x80x9d state. A xe2x80x9cwound upxe2x80x9d PID controller can no longer affect the value of the process variable because the output of the PID controller is outside the operating range of the operational unit. For example, a valve may be fully open but the xe2x80x9cwound upxe2x80x9d PID controller is asking for the valve to be five hundred percent (500%) open. For an additional example, a motor may be operating at is maximum speed of five hundred revolutions per minute (500 RPM) but the xe2x80x9cwound upxe2x80x9d PID controller is asking for the motor to run at three thousand revolutions per minute (3,000 RPM).
When the sign of the error changes, the PID controller must xe2x80x9cunwindxe2x80x9d (i.e., cease causing an excessive output signal) before the output of the PID controller returns into the proper operating range. The process of xe2x80x9cunwindingxe2x80x9d may result in xe2x80x9covershootsxe2x80x9d in the value of the process variable or may result in significant oscillations in the value of the process variable.
To prevent a PID processor from entering the xe2x80x9cwound upxe2x80x9d state it is possible to limit the contribution of the integral value when it is determined that the integral value contribution would cause the output signal to increase in the direction that will cause violation of the output limits. Implementing integral value limits in a PID controller is relatively simple because the upper and lower output limits are known, and the PID controller is able to determine whether the sum of the proportional value contribution (the xe2x80x9cP contributionxe2x80x9d) and the derivative value contribution (the xe2x80x9cD contributionxe2x80x9d) violates the output limits. If the sum of the P and D contributions do not violate the output limits, then a portion of (or all of) the integral value contribution (the xe2x80x9cI contributionxe2x80x9d) may be included in the output signal up the level of the output limit. As will now be explained, this method is not sufficient in cases involving two coupled PID controllers.
Two PID controllers may be coupled to operate in a cascade structure. In such an arrangement, the primary PID controller sends an output signal to an input of the secondary PID controller. The primary PID controller also receives a feedback signal from the secondary PID controller. The primary PID controller performs a PID calculation to determine the output signal that it transfers to the secondary PID controller. The secondary PID controller is capable of determining that the output signal of the primary PID controller has exceeded an output limit for output signals that the secondary PID controller will transfer.
The method of limiting the integral value contribution described above for the case of a single PID controller is not sufficient in the case of two coupled PID controllers because (1) the output limits in the secondary PID controller are not available to the primary PID controller, and (2) the secondary PID controller may have two different types of output limits. Specifically, the secondary PID controller may have either setpoint limits or output limits (or both types of limits). It is possible to transfer setpoint limits from the secondary PID controller to the primary PID controller as constant values. But it is not possible to transfer the output limits of the secondary PID controller as constant values. In general, when integral value calculations are involved, the PID calculation algorithm of the primary PID controller cannot determine the output limits of the secondary PID controller without complete knowledge of the past history of the input values.
One prior art method limits the integral value contribution in a primary PID controller (that is coupled to a secondary PID controller) by including or excluding the integral value contribution in response to information received from the secondary PID controller via limit flags. This prior art method causes the secondary PID controller to set an Integral High Limit Flag when the secondary PID controller has determined that its upper output limit has been exceeded. The secondary PID controller then sends information to the primary PID controller on a feedback signal line stating that the Integral High Limit Flag has been set. The secondary PID controller will not transfer the signal at the level that it received it from the primary PID controller. Instead, the secondary PID controller transfers its output signal at its normal output high limit.
The primary PID controller is capable of determining that the Integral High Limit Flag has been set by the secondary PID controller. Because the Integral High Limit Flag has been set, the primary PID controller will not include the integral value contribution in the next PID calculation. This may be done by subtraction or by multiplying the integral value by a scale factor of zero (xe2x80x9c0xe2x80x9d).
Thus, the next PID calculation will be one without any integral value contribution. The signal created by this PID calculation is usually within the range of outputs that is acceptable to the secondary PID controller. The secondary PID controller then transfers this output signal.
Because this most recent signal does not exceed the secondary PID controller""s output limit, the secondary PID controller may reset the Integral High Limit Flag to zero. The secondary PID controller then sends information to the primary PID controller on a feedback signal line stating that the Integral High Limit Flag has been reset to zero. Because the Integral High Limit Flag has been reset to zero, the primary PID controller will include the integral value contribution in the next PID calculation. This usually results in the next PID calculation causing the next output signal to once again exceed the upper output limit of the secondary PID controller""s output.
The steps described above continue to be repeated in a cycle until the PID calculations of the primary PID controller create a signal that falls within the acceptable output signal limits of the secondary PID controller.
This is an undesirable feature because it can cause a system response that swings back and forth between levels that are too high and levels that are too low. For example, this can cause an operational unit such as a valve to repeatedly open and close very quickly. It could also cause an operational unit such as a motor to repeatedly turn off and on very quickly. The erratic output signals caused by this method of limiting the integral value contribution cause the performance of the control system to suffer.
There is therefore a need for improved systems and methods for limiting the integral value contribution in a PID calculation in PID controllers that are coupled in a cascade configuration.
The purpose of the present invention is to provide improved systems and methods for limiting the integral value contribution to a PID calculation in a primary PID controller that is coupled in cascade with a secondary PID controller in order to avoid the undesirable erratic output signals that are created by using prior art methods. The method of the present invention makes it possible to prevent unnecessary wear and tear on the operational units that would otherwise have to respond to erratic output signals.
The present invention utilizes (1) a previous value of an output signal of the primary PID controller, or (2) a feedback signal from the secondary PID controller in order to determine whether to limit the integral value contribution in the next PID calculation.
The systems and methods of the present invention may be used in any type of process control system comprising a primary PID controller for controlling a first process variable coupled to a secondary controller (which may or may not be a PID controller) for controlling a second process variable. In an advantageous embodiment of the present invention the secondary controller is a PID controller. The secondary controller, however, may be an analog output unit or may be any type of controller that has setpoint limits or output limits (or both) and that is capable of setting limit flags and sending feedback signals as a PID controller does. In the description that follows the secondary controller will be referred to as a secondary PID controller. But it is to be borne in mind that the secondary controller may also be a non-PID controller.
When a primary PID controller is coupled in cascade with a secondary PID controller, the primary PID controller sends an output signal to the secondary PID controller and the secondary PID controller sends a feedback signal to the primary PID controller. The secondary PID controller is capable of determining that the output signal of the primary PID controller has exceeded a setpoint signal limit. The setpoint signal limit may be an upper setpoint signal limit, or a lower setpoint signal limit. It is also possible that the secondary PID controller will simultaneously use both an upper setpoint signal limit and a lower setpoint signal limit.
The primary PID controller performs a PID calculation to determine the output signal that the primary PID controller sends to the secondary PID controller. The PID calculation is the sum of a proportional calculation component and an integral calculation component and a derivative calculation component.
When the secondary PID controller determines that a previous output signal of the primary PID controller has exceeded a setpoint signal limit, it becomes necessary for the primary PID controller to make adjustments to the next output signal that the primary PID controller sends to the secondary PID controller. This requires the primary PID controller to make adjustments to the next PID calculation. The present invention provides improved systems and methods for limiting the contribution of the integral calculation component to such a PID calculation.
The present invention limits the contribution of the integral calculation component in a PID calculation by multiplying the integral calculation component by zero in response to a determination that the current sum of a proportional calculation component and a derivative calculation component of the PID calculation exceeds a previous value of an output signal of the PID controller. Equivalent to multiplying the integral calculation component by zero, the entire integral calculation component may simply be excluded from the PID calculation.
The present invention also limits the contribution of the integral calculation component in a PID calculation by multiplying the integral calculation component by a non-zero scale factor having a value between zero and one in response to a determination that the inclusion of the integral calculation component in the current PID calculation would otherwise cause the current value of the output signal of the PID controller to exceed a previous value of the output signal of the PID controller. Equivalent to multiplying the integral calculation component by a non-zero scale factor, the portion of the integral calculation component contributing the excess value of the output signal may simply be excluded from the PID calculation.
It is an object of the present invention to provide improved systems and methods for limiting the contribution of an integral calculation component in a PID calculation in a control apparatus having an upper output signal limit.
It is also an object of the present invention to provide improved systems and methods for limiting the contribution of an integral calculation component in a PID calculation in a control apparatus having a lower output signal limit.
It is an additional object of the present invention to provide improved systems and methods for limiting the contribution of an integral calculation component in a PID calculation in a control apparatus having both an upper output signal limit and a lower output signal limit.
It is an object of the present invention to provide improved systems and methods for limiting the contribution of an integral calculation component in a PID calculation in a control apparatus having an upper setpoint signal limit.
It is also an object of the present invention to provide improved systems and methods for limiting the contribution of an integral calculation component in a PID calculation in a control apparatus having a lower setpoint signal limit.
It is an additional object of the present invention to provide improved systems and methods for limiting the contribution of an integral calculation component in a PID calculation in a control apparatus having both an upper setpoint signal limit and a lower setpoint signal limit.
It is a further object of the present invention to provide improved systems and methods for avoiding undesirable erratic output signals that are present in prior art PID controllers.
It is an additional object of the present invention to provide improved systems and methods for limiting the contribution of an integral calculation component in a PID calculation in a control apparatus having both output signal limits and setpoint signal limits.
It is another object of the present invention to provide improved systems and methods for preventing unnecessary wear and tear in operational units that have to respond to the erratic output signals that are present in prior art PID controllers.
The foregoing has outlined rather broadly the features and technical advantages of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features and advantages of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they may readily use the conception and the specific embodiment disclosed as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.